It is currently predominantly microlithographic projection exposure methods which are used to produce semiconductor components and other finely structured components. In this case, use is made of masks (reticles) which carry or form the pattern of a structure to be imaged, for example, a line pattern of a layer of a semiconductor component. A mask is positioned in a projection exposure machine between an illumination system and a projection objective in the region of the object surface of the projection objective, and illuminated in the region of an effective object field with the aid of an illuminating radiation provided by the illumination system. The radiation varied by the mask and the pattern passes as an imaging beam through the imaging beam path of the projection objective, which images the pattern of the mask in the region of the effective image field optically conjugate to the effective object field onto the substrate to be exposed. The substrate normally carries a layer (photoresist), sensitive to the projection radiation. One of the aims in the development of projection exposure machines is to lithographically produce structures with increasingly smaller dimensions on the substrate. Relatively small structures lead to relatively high integration densities in the case of semiconductor components, for example, and this generally has an advantageous effect on the performance of the microstructured components produced. The size of the structures which can be produced depends on the resolving power of the projection objective used, and can be increased, on the one hand, by reducing the wavelength of the projection radiation used for the projection and, on the other hand, by increasing the image-side numerical aperture NA of the projection objective which is used in the process.
It is predominantly purely refractive projection objectives which have been used in the past for optical lithography. In the case of a purely refractive or dioptric projection objective, all the optical elements which have the refractive power are transparent refractive elements (lenses). In the case of dioptric systems, it becomes more difficult with rising numerical aperture and falling wavelength to correct elementary aberrations such as, for example, to correct chromatic aberrations and to correct the image field curvature.
One approach to attaining a flat image surface and a good correction of chromatic aberrations is to use catadioptric projection objectives which include both refractive transparent optical elements with refractive power, that is to say lenses, and reflective elements with refractive power, that is to say curved mirrors. At least one concave mirror is typically included. The contributions of lenses with positive refractive power and lenses with negative refractive power in an optical system are respectively opposed to the total refractive power, the image field curvature and the chromatic aberrations. However, just like a positive lens, a concave mirror has a positive refractive power, but a concave mirror has an effect on the image field curvature inverse to a positive lens. Moreover, concave mirrors do not introduce any chromatic aberrations.
As a rule, projection objectives have a multiplicity of transparent optical elements, in particular positive lenses and negative lenses, in order to enable partially opposite properties with regard to the correction of aberrations even given the use of large numerical apertures. Both refractive and catadioptric imaging systems in the field of microlithography frequently have ten or more transparent optical elements.
The optical elements are held with the aid of holding devices at defined positions along an imaging beam path. In the field of optical systems for microlithography, very complex technologies have been developed in this case for the design of holding devices in order to ensure a high imaging quality of the imaging system by exact positioning of the held optical elements in the imaging beam path in conjunction with different operating conditions and in order to ensure that the expensive and sensitive optical elements are held without stress in as gentle a manner as possible. In the field of microlithography, lenses and other transparent optical elements are frequently supported via a multiplicity of holding elements which are arranged uniformly on the circumference of the respective optical element. In this case, the optical element has an optical useful region lying in the imaging beam path, and an edge region lying outside the optical useful region. One or more holding elements of the holding device are assigned to the optical element acting on the edge region in the region of a contact zone. The surfaces of the optical element are prepared with optical quality in the optical useful region, whereas the optical quality need not be reached in the edge region. The optical useful region is frequently also denoted as the “free optical diameter” of the optical element.
Different options have already been proposed for fixing the optical element on the holding elements. Patent application US 2003/0234918 A1 exhibits examples of clamping technology in which an optical element is held in the edge region of elastomeric holding elements which clamp the optical element in the region of a respective contact zone in the edge region and permit overall a certain mobility for the optical element being held (soft mount). With other holding devices, elastic holding elements of a holding device are bonded to the optical elements in the region of the respectively assigned contact zone. Examples of the bonding technology are shown in U.S. Pat. No. 4,733,945 or U.S. Pat. No. 6,097,536.
In addition to the intrinsic aberrations which a projection objective can have owing to its optical design and its production, aberrations can also occur during the service life, for example during operation of a projection exposure machine. Such aberrations frequently find their cause in changes to the optical elements installed in the projection objective resulting from the projection radiation used during operation. For example, this projection radiation can be partially absorbed by the optical elements in the projection objective, the extent of the absorption depending, among other things, on the material used for the optical elements, for example the lens material, the mirror material and/or the properties of antireflection coating possibly provided, or reflective coatings. The absorption of the projection radiation can lead to heating of the optical elements and this can result, directly and indirectly, in surface deformation of the optical elements and, in the case of refractive elements, a change in refractive index via thermally caused mechanical stresses. Changes in refractive index and surface deformations lead, in time, to changes in the imaging properties of the individual optical elements, and thus also in the projection objective overall. This range of problems is frequently treated under the heading of “lens heating”.
An attempt is usually made to compensate thermally induced aberrations or other aberrations occurring during service life at least partially by using active manipulators. As a rule, active manipulators are optomechanical devices which are set up to influence individual optical elements or groups of optical elements on the basis of corresponding control signals, in order to change their optical action so that an aberration which occurs is at least partially compensated. By way of example, it can be provided for this purpose that individual optical elements or groups of optical elements are deformed, or their positions are changed.
Such active manipulators are frequently integrated in the mounting technology, that is to say in the holding device. Relating to this, for example, US 2002/0163741 A1 shows a holding device for a transparent optical element designed as a lens. The holding device has an inner hexapod structure which passively couples an inner ring to the lens, and an outer hexapod structure which functions as a controllable manipulator. Fitted on the inner ring are three clamping devices which are uniformly distributed within the circumference and act in a clamping fashion on the edge of the lens and fix the latter in the inner ring.
In practice, with optical imaging systems of complex design, not only does the radiation pass from the object into the image plane through the imaging beam path desired for the imaging, but it is also possible for radiation components to be produced which not only contribute to the imaging, but disturb and/or impair the imaging. For example, in the course of projection exposure methods so called “over-apertured light” can pass through the substrate to be exposed, for example, a semiconductor wafer, and disturb individual wavefronts. The term “over-apertured light” or “superaperture radiation” here denotes radiation which is diffracted by the structuring mask and emitted at an angle which is larger than the object-side aperture angle used for the imaging and is determined by the current diameter of the aperture stop limiting the imaging beam path. For reasons of production technology, such over-apertured light can nevertheless fall onto the image plane of the imaging system through the aperture stop. Since, as a rule, the imaging of the optics is not designed, and correspondingly not completely corrected, for apertures larger than a maximum usable aperture, this can severely disturb the wavefront contributing to the imaging, and so the imaging quality can be impaired by over-apertured light. Alternatively, or in addition, it is also possible for scattered light to be produced which generally impairs the contrast of the image produced when it passes as far as into the image plane. The term “scattered light” denotes here, among other things, radiation which can, for example, result from residual reflections at the surfaces, coated with antireflection layers, of transparent optical elements, on the rear sides of mirrors and/or at other points in the region of the imaging beam path. These undesired radiation components, in particular, the scattered light and the over-apertured light, are also denoted as “false light” in the context of this application irrespective of their cause.
False light which does not reach the region of the image field in the image plane and thereby does not directly disturb the imaging can nevertheless disadvantageously affect the quality of an imaging process when it impinges on points of the system not provided for the irradiation and is possibly absorbed there. False light can, for example, be absorbed in parts of the mounting of the optical imaging system and generate heat which reacts to affect the position and/or shape of the optical elements via corresponding thermal expansion of affected components. False light can also be absorbed by parts of the imaging system which consist neither of a transparent optical material nor of metal. Examples of this are cables, sensors and/or actuators of manipulator devices which can include plastic parts which outgas when irradiated. This outgassing is generally undesired, since not only can the functioning of these parts be impaired thereby, but also because the atmosphere inside the imaging beam path can be influenced.
It is known for optical imaging systems of complex design, such as projection objectives for microlithography for example, to be equipped with one or more baffle plate plates in order to reduce the negative effects of scattered light on the imaging. For example, patent U.S. Pat. No. 6,717,746 B2 gives exemplary embodiments of catadioptric projection objectives in which a baffle plate for reducing scattered or false light can be inserted in the region of a real intermediate image formed between the object plane and image plane. The international patent application with publication number WO 2006/128613 A1 shows examples of catadioptric projection objectives with a first objective part for imaging an object in a first real intermediate image, a second objective part for producing a second real intermediate image with the aid of the radiation coming from the first objective part, a third objective part for imaging the second real intermediate image in the image plane, and a radiation directing device which ensures that the radiation in the second objective part runs in another direction than in the first and third objective parts. At least one diaphragm is arranged in the region of the beam deflection device such that that a radiation component which would run in a fashion bypassing the second objective part directly from the first objective part to the third objective part is effectively reduced by screening. In this way, what passes into the image plane is predominantly or exclusively radiation which uses the entire imaging beam path with all the optical components provided therein and can, correspondingly, contribute to the imaging with a well corrected wavefront.